Analytical method: QCM06801 - Manganese determination in MnO

1. Analytical technique

Manganese assay is carried out by potentiometric method.

2. Summary

1. Dissolution of the needed quantity of product in HC1 with gentle warming.
2. Oxydation of Fe II ion by adding HNO3, bring to boiling.
3. Add urea to eliminate inferior oxides.
4. Potentiometric determination of the ion Mn2+ by oxidation in Mn3+ by using potassium
permanganate in neutral medium in the presence of sodium pyrophosphate (complexing
agent for Mn3+) according to the following reaction :
MnO-4 + 4 Mn2+ + 8 H+ + 15 H2P2O7 =5Mn (H2P2O7)33- + 4 H2O
The equilibrium is entirely shifted to the right at pH 6,5 - 7,0 due to the formation of the complex.

3. Equipment

1. Equipment for potentiometric titration (equipped with a combined platine electrode
reference Ag/Ag Cl - KC1 3 M) with a pHmeter (KC1 3 M electrode).
2. Analytical balance with an accuracy of 0,1 mg.
3. Laboratory glassware.
4. Hot plate.
5. Magnetic stirring plate with stirring bars.
6. Storage and reaction flasks including 2 bottles in brown glass.
7. Water bath for cooling.
8. Acid dispenser.
9. Filter funnel with glass frit G3.

4. Reagents

1. Hydrochloric acid p.a. (d = 1.18).
2. Nitric acid p.a. (d = 1.40).
3. Crystalline urea p.a.
4. Sodium pyrophosphate p.a.
5. Potassium permanganate.
6. Metallic manganese min. 99.99 percent purity.
7. Demineralized water.
8. KC1 3M for the electrode (ready to use solution).

5. Preparation of reagent solution

1. Diluted hydrochloric acid solution (6M): dilute 500 ml concentrated HC1 in 1000 ml.
2. Saturated solution of sodium pyrophosphate: dissolve in water 67 g of Na4.P2.O7.10H2O
in 900 ml pure water at the temperature 35°C with stirring. Let it cool down at room
temperature and bring the volume to one liter, homogenize and transfer to a storage
flask. (The preparation of this solution does not require calibrated glassware).
3. Potassium permanganate solution: to a 1250 ml beaker, add 1000 ml distillated water.
Add with stirring (magnetic stirring) 3,0 g of KMnO4 p.a. and allow to dissolve with a
gentle warming. Let it cool down at room temperature and bring the volume to one
liter. Allow to settle for 24 hours in the dark. Filter the solution on a glass frit G3.
Transfer the solution to a storage flask of brown glass and homogenize completely.
4. Standard solution of manganese at 5,00 g/l : weigh 5,00 g of metallic manganese of
99.99 % purity. Transfer this to a 400 ml high form beaker provided with a watch glass
cover. Add 30 ml concentrate hydrochloric acid. Allow to digest at room temperature
till all the solid is dissolved. Carefully rinse the watch cover and the walls of the beaker,
then transfer quantitatively to 1000 ml volumetric flask. Bring to level with distillated
water, homogenize completely and transfer to a P.E. storage bottle.

6. Standardization of potassium permanganate solution

1. Take 10 ml of the standard solution of manganese by means of calibrated transfer
pipette and add this aliquot to a 250 ml beaker.
2. Repeat this three times.
3. Add 20 ml distillated at each sample.
4. Add 100 ml of sodium pyrophosphate and stir with a magnetic agitation.
5. Bring the solution to a pH 6.7 by adding diluted hydrochloric acid solution.
6. Proceed with the titration using potassium permanganate (to a potentiometric endpoint
at 440 mv).
Take notice that it is important to maintain the pH between 6.5 and 7.0 with either dilute HC1 or NaOH throughout the course of titration.
7. The determinations should be run and the average is taken.
The determinations are correct if a difference between the three samples is not greater than 0.002; otherwise repeat from the beginning.
Calculation and expression of the titre of the potassium permanganate solution
The titre of the solution of KMnO4 is defined as a quantity of manganese expressed in mg reacting with exactly 1 ml of KMnO4. This titre is usually called "manganese factor" of the potassium permanganate solution.
Let put V1 the number of ml of KMnO4 utilized. The quantity of manganese present in the aliquot solution, which was taken, is 50 mg.
FMn = 50/V1

7. Sample preparation

1. Weigh (exactly) about 0,2 g of the product and introduce this quantitatively into a
250 ml beaker provided with a magnetic stirrer.
2. Then add 10 ml distillated water and 1 ml concentrated hydrochloric acid; cover the
beaker with a watch glass.
3. Heat the sample until the dissolution is complete. Allow the solution to cool a bit.
4. Add 2 or 3 drops of concentrated nitric acid and bring to boiling. Continue to boil the
solution just to the point where nitric acid vapors are obtained but without allowing to
go to dryness.
5. Allow to cool several minutes, carefully rinse the watch glass and the wall of beaker and
add a few grms of urea.

8. Analysis of the sample

1. Add 190 ml of the sodium pyrophosphate solution.
2. Agitate with a mechanical stirrer and adjust pH to about 6.7 with NaOH.
3. Proceed immediately to the titration by mean of potassium permanganate; the pH being
maintained between 6.5 abd 7.0 throughout the determination period by means of NaOH
or HC1 addition.
4. Endpoint is found at 440 mv.

Content of manganese is expressed in percent and is calculated by the formula:
(V2 x FMn x 100)/(x 1000)

V2 : KMnO4
FMn : manganese factor of the KMnO4 solution
P : weight of the unknown sample